Non-volatile semiconductor memory devices are memory devices that can preserve stored data even when power is not supplied to the device. Various types of memory cells that are appropriate for non-volatile semiconductor memory devices are known. One such type of memory cell is the single transistor type memory cell.
A conventional single transistor type memory cell MC is illustrated in FIG. 1. As shown in FIG. 1, the memory cell MC includes a current path formed between a source S and a drain D on a semiconductor substrate, a floating gate FG formed between a dielectric oxide DOX and a gate oxide GOX, and a control gate CG. The floating gate FG traps electrons, and the trapped electrons establish the threshold voltage of the memory cell MC. To perform a read operation on the memory cell MC, the threshold voltage of the memory cell MC is detected and the value of the data is determined based on the detected threshold voltage level.
Most non-volatile semiconductor memory devices include memory cells MC that can be programmed and erased repeatedly. A single transistor memory cell MC is programmed by trapping electrons in the floating gate FG. The electrons may be trapped in the floating gate FG by, for example, Fowler-Nordheim tunneling (FN) or electron injection. Electron injection may be performed, for example, by Channel Hot-Electron injection (CHE) or Channel-Initiated Secondary Electron Injection (CISEI). Fowler-Nordheim tunneling is widely used in flash memory devices in which data is erased simultaneously.
Typically, a single transistor memory cell MC stores one of two data values. As shown in FIG. 2, the two data values are determined by a threshold voltage that is set to one of two levels. For example, data may be read as “1” when the detected threshold voltage of the memory cell MC is lower than a reference voltage VM, and data may be read as “0” when the detected threshold voltage of the memory cell MC is higher than the reference voltage VM.
A four-level memory cell has been developed that can provide for increased integration of semiconductor memory devices. The four-level memory cell, as illustrated in FIG. 3, can be programmed with one of four threshold voltage levels. As a result, the four-level memory cell can store any one of four data values. Therefore, a non-volatile semiconductor memory device having four-level memory cells (hereinafter referred to as a “four-level non-volatile semiconductor memory device”) has a data storage capacity that is twice that of a non-volatile semiconductor memory device having two-level memory cells (hereinafter referred to as a “two-level non-volatile semiconductor memory device”).
In a four-level memory cell, the margin of the threshold voltage between neighboring levels typically is very narrow (e.g., about 0.67 V). Moreover, the threshold voltage of each memory cell may be shifted due to the leakage of electrons or the like. Accordingly, the threshold voltage of the memory cell MC may be shifted from a threshold voltage programmed with one of the four levels to a neighboring level threshold voltage, which can result in errors in reading the data stored in the four-level memory cell.